One definition of "universe" is everything that exists. Another definition says a "universe" is everything that exists and is connected (so there can be several universes that don't interact with each other.)

If I create a fantasy world in my head, the people and places exist in my head; if I create a fantasy world in my computer, the people and places exist on my computer. Thus, at least in some sense, they exist. In a way, everything within the simulation makes up a universe; in another way, it's more like a sub-universe that has internal interactions as well as external interactions with the simulation hardware. From the perspective of a simulated character who doesn't know about the computer or my brain, they live in their own universe.

(P.S. are you the Suncho from the Descent world? If so, it's good to see you again.)

Yes. One definition of "everything that exists" is "everything that exists and is connected." We can think of the universe as a directed graph and "everything that exists" as nodes that are reachable from where we're standing. From within a sub-universe, "everything that exists" is a subset of "everything that exists" for someone standing in a containing universe.

(P.S. Yes. Hi Lothar. Small world.)

I want to believe in universe is a computer simulation, because that way all because less serious. What disturbs me is that, in history, people usually tried to explain the existence by using the most sophisticated machinery available. and today it is a computer.....are we making a mistake?

If your goal is to understand the universe around you, wanting to believe something can be a mistake. Similarly, trying to explain the cause of existence can also be a mistake. Use the simplest explanation that fits your observations. I find it helpful to start from solipsism and build from there.

so, the universe is a room.

Is that a Sapphire and Steel reference?

was not, but I now learned about Sapphire and Steel, thanks to your comment.